PISCES 



197 



Size and Number. — Fishes vary in length from an inch or less 

 to 30 or 40 feet. Kellogg says there are about "15,000 species 

 of fishes known, of which 3000 species live in North America." 

 Is it any wonder they vary in size, color, and habits? They are 

 the best adapted of all vertebrates for an aquatic life. 



Covering. — The epidermis consists of many layers of proto- 

 plasmic cells with a very thin cuticle. The secretion of mucus 

 by the great numbers of "slime-cells" of the epidermis gives to 

 fishes their slippery skin. The epidermis contains also pigment 

 cells. The dermis consists of numerous layers of connective 



Fig. 158. — Figure of a whitefish, showing the location of parts usually- 

 referred to in descriptions: 1, Dorsal fin; 2, adipose fin; 3, caudal fin; 4, 

 anal fin; 5, pectoral fin; 6, ventral fin; 7, lower jaw, or mandible; S, upper 

 jaw, or maxillary; 8a, suppleinciilal luaxillary; 9, opercle; 10, branchioste- 

 gals; 11, caudal peduncle; 12, lateral line; 1.3, series of crosswise scales usually 

 counted; 14, snout; 15, eye; 16, head; 17, depth; 18, base of caudal; 19, dis- 

 tance from snout to nape or occiput. (Report U. S. F. C, 1894.) 



tissue and furnishes the dermal or exoskeleton or scales which are 

 usually embedded in pockets of the dermis. 



In many fishes the scales overlap each other. In the brown 

 trout, for example, the greater portion of each scale lies under 

 the one anterior to it, and the remainder, a small triangular por- 

 tion, is covered by the epidermis only. The scales sometimes 

 receive a layer of enamel or vitrodentin from the epidermis. 

 (1) The placoid scales are rhombic, plate-like bodies often bearing 

 a spine covered with vitrodentin. These scales are placed close 

 together, but do not overlap. (2) The ganoid scales also are 



